It must be possible in all cellular systems to control at least the transmission power of the personal station for its transmission to arrive at the base station with a sufficient signal-to-noise ratio irrespective of the distance between personal station and base station. Power transmission will be explained in the following using the CDMA system (Code Division Multiple Access) as an example. FIG. 1 shows a CDMA forward traffic channel. This comprises the following code canals: a pilot channel, one synchronization channel, from one to seven paging channels and no more than 61 traffic channels. The maximum number is when there is only one paging channel besides the synchronization channel. Each code channel is orthogonally hashed and thus spread out by using the phase quadrature couple of the random noise sequence. At the base station several forward traffic CDMA channels may be used by way of frequency-divided multiplexing. The structure shown in FIG. 1 is presented in Proposed CDMA PCS Standard, Joint Technical Committee (JTC), Sep. 23, 1994. This proposal is also known by the name IS-95. Reference will be made in the following to a CDMA system according to this standard, although the invention is suitable for any kind of system.
An unmodulated spread spectrum signal is sent continuously on the pilot channel and it is used for synchronization by the PS (Personal Stations).
An encoded, interleaved, spread and modulated spread spectrum signal is sent on the synchronization channel. The personal station uses this signal for achieving a preliminary time synchronization. The channel bit rate is 1200 bps and the frame duration is 26,666 ms. No sub-channel relaying power control commands must be included in the synchronization channel.
An encoded, interleaved, spread and modulated spread specturm signal is sent on the paging channel. The data rate is 9600 or 4800 bps and the frame duration is 20 ms. The base station uses the paging channel for transmit overhead information and personal station specific information. The number of these channels may vary in one CDMA forward traffic channel, however, the maximum number is 7 channels.
The traffic channel is used for transmitting user and signaling information to the PS (Personal Station). The maximum number of simultaneous forward traffic channels supported by one CDMA traffic channel is 63 minus the number of call and synchronization channels operating on one and the same CDMA traffic channel.
The frame structure itself is the same both on the forward traffic channel and on the reverse traffic channel. The information is transmitted as frames, the length of which is 20 ms. The base station and the personal station may send information at a varying data rate. Data transfer rates when using rate set 1 are 9600, 4800, 2400 and 1200 bps respectively while the corresponding frame bit numbers at different rates are 192, 96, 48 and 24 bits respectively. When using rate set 2, data transfer rates are 14400, 7200, 3600 and 1800 bps respectively while the corresponding frame bit numbers are 288, 144, 72 and 36 bits. The frame bits are formed by information bits, frame quality indicator bits and encoder tail bits. The essential thing is that in both directions the structure of the traffic frame is different at different transfer rates, so when identifying the frame structure the transfer rate will also be known.
Such modulation symbols which are transferred at a lower data rate are also transmitted with a lower energy, but although the data rate varies from one frame to another, the symbol modulation rate is kept constant. When entering Es energy per symbol and Eb energy per information bit, the following Table 1 according to the standard will apply:
two successive modulation symbols are removed at regular intervals and they are replaced with a power control bit. Thus, the time width of one power control bit is 104.166 ms. The procedure is generally known in the field and it is called symbol puncturing. The puncturing figure shows which symbols are removed from the frame and replaced with power control bits. The power control bit is transmitted with energy Eb.
Having received the power control bit, the personal station will increase or decrease its transmission power in the direction indicated by the bit. The power control bit is considered genuine, if it was received in that 1.25 ms time slot, which is the second time slot counting from the time slot in which the personal station has transmitted. The change of power level is a small step, and the standard determines that one bit changes the power level by 1 dB. A great change of the power level will thus require transmission of several power control bits.
As is done in FDD/TDMA systems, DTX (Discontinuous Trans-mission) is also used in CDMA systems. In a broader sense, DTX also includes an asymmetric case where information is transferred in one direction only, while acknowledgements are transferred in the opposite direction. An Internet connection is an example of such a situation. Power control commands are sent to the receiving party at a normal frequency even if the party is sending information only occasionally.
It is possible to go over to the DTX state in different ways. Firstly, when the personal station discovers that the data transfer rate which it needs is dropping, it will first send to the base station information on the data rate to be used in the following radio frame and then from the next frame forward it will use the rate which it stated. Secondly, the personal station may change the transfer rate during the connection without further ado. The base station will learn the transfer rate from the frame structure, because, as was said above, when the frame structure is identified the used transfer rate will also be known, since the frame structure varies at different transfer rates.
It is a problem in the presented CDMA system and also in other known CDMA systems that power control commands of one or several bits are always sent at the same standard frequency and energy. Power control is fast so that the transmission power will comply as closely as possible with changes in the radio path. In addition, power control commands are sent both in the forward direction and in the reverse direction at the same
TABLE 1Data rateEnergy per modulation symbol9600Es = Eb/24800Es = Eb/42400Es = Eb/81200Es = Eb/1614400 Es = Eb/47200Es = Eb/83600Es = Eb/161800Es = Eb/32
Each forward traffic channel contains a power control sub-channel, which is used for transmitting such power control commands to the personal station during the communication, in response to which the personal station will change its transmission power. The power control channel is described in item 3.1.3.1.8 of the specification proposal.
From its received personal station signal the base station calculates the signal power always with intervals of 1.25 ms, which time corresponds to 16 modulation symbols. Judging by the signal power the base station will command the personal station to increase or decrease the transmission power. Thus a big power control loop is formed, which comprises the personal station, the base station and the two-way radio channel in between. The base station may also control its own transmission power to correspond with the power measurement reports which it receives from the personal station. This is so because the personal station constantly keeps statistics on frame errors and sends the power measurement report regularly or when a certain threshold value is exceeded.
The power control sub-channel is formed in such a way that power control bits are sent constantly among the normal traffic channel bits. The power control bits are repeated cyclically at intervals of 1.25 ms. Hereby the bit rate of the power control channel is 800 bps. Bit 0 means that the personal station must increase its transmission power, and correspondingly bit 1 means a command to lower the transmission power. The bits are located in the frame so that from the completed traffic frame, which is a convolution coded and interleaved frame formed from modulation symbols, frequency and the power control will thus not be affected by the transfer rate used, by asymmetry of the data transfer or by the fact that either party may be in the DTX state. The outcome is, that in the DTX state and when using a reduced data transfer rate, the power control will use a disproportionately large share of the radio link's capacity.
It is thus an objective of the present invention to bring about a method of power control which adapts to the data transfer situation, releasing radio link capacity for other use in the DTX state and at a reduced data transfer rate.
The objective is achieved with the method defined in the independent claims.